Upload docker image to docker hub (from master)

[certik]

Travis should upload the docker image to docker hub, only from master (it should ignore PRs for this). Alternatively, perhaps easier, I think the docker hub itself can do that for us:

https://docs.docker.com/docker-hub/builds/

So we should just set it up.

That should simplify deployment, as one doesn't have to build the image, only pull it.

[certik]

Ok, I think it's setup:

https://hub.docker.com/r/certik/planet-sympy/

That was very simple.

[certik]

I will keep this issue open for a while, to ensure it is working as we update our master a few times.